In one type of air operated grease gun previously available, a reciprocating plunger ejects grease from the tip of the gun. The plunger is driven by a piston reciprocating in a cylinder. Air pressure in the cylinder advances the piston in the cylinder against the force of a return spring, until the piston reaches a position where a release opens a seal over apertures in the piston, allowing the air pressure to escape and the piston to retract under the force of the return spring.
This type of air operated grease gun suffers from at least two disadvantages. The opening of the seal results in a significant pressure drop in the pressure of the grease ejected by the gun, resulting in large swings in the grease application pressure. Furthermore, the piston must be relatively large and rigid, and carefully manufactured to close tolerances so that the seal can close all of the apertures so that the piston functions properly. This makes the piston very expensive, increasing the total cost of the grease gun.